Method and apparatus for remotely controlling a seismic vibrator and recording system

ABSTRACT

A seismic vibrator capable of producing a computer controlled signal of predetermined frequency characteristics is remotely initiated using encoding and decoding devices. A similar recording apparatus is simultaneously placed into operation.

- United States Patent Pelton et a1.

[ June 19, 1973 [75] Inventors: Charles R. Pelton; Kim L. Mitchell,

both of Ponca City, Okla.

[73] Assignee: Pelton Company, Inc., Ponca City,

Okla.

3,219,971 11/1965 Cole 340/15 5 3,460,648 8/1969 Brown et a1. 340/15.53,440,599 4/1969 Waters et a1 340/155 Primary ExaminerBenjamin A.Borchelt Assistant Examiner-J. V. Doramus Attorney-Head & JohnsonDETECTOR [22] Filed: Jan. 29, 1971 [2l] Appl. No.: 110,872

- [57] ABSTRACT [52] U.S. Cl. 181/.5 FS, 340/155 R 51 It.Cl. 01 116 01122 E 0 Search v Q A seismic vibrator capable of producing a computer340/155 55 b 3 controlled signal of predetermined frequency charac- 1 5teristics is remotely initiated using encoding and decoding devices. Asimilar recording apparatus is simulta- [56] References Cited neouslyplaced into operation.

UNITED STATES PATENTS 2,688,124 8/1954 Dory et a1. 340/155 12 Claims, 5Drawing Figures 42 I A! 34 ENcoDING ANALDG- DIGITAL CONTROL m /4 A 46SIGNAL GENERATOR 45 /a RECORDING ENCODING Q} I PROCESSING l RADIO 35SECTION EQUIPMENT T G NG g RADIO k 5 f 44 52 DEcoDING ANALOG;

DIGITAL CONTROL SIGNAL 54 GENERATOR -coNTRoL sIGNAL GENERATING SECTIONCONTROL ELECTRONICS Pmmtnav $739370 S'EEI 1 II 5 mobm QM @3855 mumINVENTOR. CHARLES R. PELTON KIM L. MITCHELL ATTORNEYS EwsESS S 3518mm wwPATENIED 9 SIEET '& If 5 INVENTOR. CHARLES R. PELTON KIM L. MITCHELLATTORNEYS BACKGROUND This invention relates to improvements in the artof geophysical prospecting, and more particularly, but not by way oflimitations, to a method and apparatus for developing simultaneously, indifferent locations, highly accurate identical control signals ofdesired frequency limits and proportions for controlling the output ofseismic vibrators and at-the same time making the signal available atanother point or points to be recorded or used in the processing of theseismic data without the use of wire or wireless to transmit the actualcontrol signal.

In seismic surveying it has been found highly desirable to impartvibrational seismic energy of predetermined characteristics into theearth from one or more vibrators. In order to enhance the value of theseismic data by using a multiplicity of vibrators, it is highlydesirable that all the vibrators impart their signals into the ground atthe proper time and phase. Much work has been done on causing thevibrator signals imparted into the earth to be unique and of a desiredphase relationship relative to the signal controlling the vibrator, Fora discussion of such background, reference is hereby made to US. Pat.Nos. 3,208,545; 3,219,971; and the correlation prospecting system asdescribed in No. 2,688,124.

Some of the art has suggested transmitting the vibrator control signalfrom a central operational position via radio-transmission receptionmeans, as for example suggested in US. Pat. No. 2,460,648 and others.However, such systems are subject to phase inaccuracy and interferencewhich degrade the overall performance and results.

Those skilled in the art known under the trademark VIBROSEIS ofContinental Oil Company have long recognized the desire to achieve avibrational control signal of predetermined variable amplitude (see G.L. Brown and B. J. Thomas paper Comparison of Digital And Analog FieldRecording And Compositing in VI- BROSEIS Exploration, presented Societyof Exploration Geophysicists, Sept, 1969, meeting Calgary, Canada).

SUMMARY The present invention describes a method and apparatus forgenerating all seismic control signals, vibrator and recording, at thedesired high degree of phase accuracy with respect to one anotherwithout regard to the presence of radio interference that would cause aconventionally transmitted control signal to be unusable.

Another object of the invention is to decrease the contamination oftheair waves by elimination of continuous radio transmission during thetime of seismic recording.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diametric view of theprior art method of conducting a seismic survey.

FIG 2 is an overall view of the method of a seismic survey utilizing theprinciples of this invention.

FIG. 3 is a schematic diagram of an encoding circuitry for use with theinvention.

FIG. 4 is a schematic diagram of the decoding circuitry section for usewith invention.

FIG. 5 is a schematic diagram of a control signal generating circuit foruse with this invention.

PRIOR ART Referring now to FIG. 1, the present means of vibrationalsurvey makes use of a recording system, generally designated 10, and avibrator system 12. The recording system 10 usually comprises recordingprocess equipment 14, a seismic vibration detector array 16 and a radio18. Detector array 16 comprises suitable geophones which are connectedto recordingprocessing equipment 14 as by means of wires 20 or othersuitable means. Radio 18 is connected to recording-processing equipment14 by wire 22.

Vibrator system 12 normally consists of a radio receiver 24 connected bywire 26 to a control circuit 28 which in turn is connected to a suitablevibrator 30 by means of wire 32. Radio 18 communicates with radioreceiver 24 via respective antennae 34 and 35.

The recording-process equipment 14 produces a control signal, eithercomputed or from storage, and sends the signal through wire 22 to radio18 which in turn transmits the signal to radio receiver 24. Radioreceiver 24 introduces the vibratory control signal to the controlelectronics 28 through wire 26 which in turn causes the vibrator by wayof wire 32 to impart the desired vibratory seismic signals into theearth 36 as is shown by one possible path 38. This seismic vibratorysignal is reflected or refracted by an interface 40 deep in the earth todetector array 16 which is coupled to the earths surface. The detectorarray produces a voltage representing the movement of the seismicsignals. This voltage is then transferred through wire 20 to therecording process equipment where it is permanently recorded andotherwise used by seismic interpreters to determine geologicalstructure.

DESCRIPTION OF PREFERRED EMBODIMENT The method of this invention differsfrom the abovedescribed system inasmuch as the vibrator control signalis not transmitted from the recording-processing equipment to thevibrator system, rather, the control signal is generated locally at eachvibrator and at the recording-processing equipment in response to acommon radio transmitted coded signal. The overall sketch of FIG. 2depicts one system for remotely controlling and activating a seismicvibrator utilizing the principles of this invention. In this method, therecording system comprises recording-processing equipment 14, detectorarray 16 and radio 18 as before. However, conduit 22, betweenrecording-processing equipment 14 and radio 18, is replaced by anencoding analog-digital control signal generator 42. Likewise thevibrator system includes the normal radio receiver 24, controlelectronics 28, and vibrator 30. However, electrical conduit 26 isreplaced by a decoding analog-digital control signal generator 44.Encoding analog-digital control signal generator 42, as the nameindicates, includes an encoding section 46 whose input is coupled torecording processing equipment 14 by wire 45. Encoding section 46 hastriple outputs; one output is coupled to control signal generator 48A byconduit 47, a second output controls radio 24 over conduit 50, and athird output carries a coded tone to radio 24 over conduit 58. Signalgenerator section 48A has an output which feeds 3 through wire 52 of therecording processing equipment 14.

Decoding analog-digital control signal generator 44 includes a decodingsection 54 having the input thereof fed from radio receiver 24 andhaving an output feeding a second control signal generator 48B which inturn feeds control electronics 28 through a wire 56.

In operation, recording-processing equipment 14 induces a start signalinto the encoding section 46. Encoding section 46 turns on radio 18through wire 50 and generates a tone of specific frequency and phasecharacteristics for the purpose of modulating radio 18 via wire 58 andfurther introduces an initiate pulse into computer section 48A. The tonesignal from radio 18 is picked up by radio receiver 24 and passed intodecoder section 54.

Decoding section 54 decodes the tone received from radio receiver 24 andpresents an initiate pulse to control signal generating section 48Bwhich computes a mathematically predetermined unique or control signalwhich, in turn, causes vibrator 30 to impart such unique vibratoryseismic signal 38 into the earth. The reflected and/or refracted signalsare subsequently picked up by detector array 16 and relayed in the formof voltage to recording-processing equipment 14. The unique signalsgenerated by control signal generating stations or computers 48A and 48Bare identical in nature and occur at the desired high degree of accuracyrelative to each other in time.

To better understand the operations of the encoding analog-digitalcontrol signal generator 42 and the decoding analog-digital controlsignal generator 44, attention will now be turned to FIGS. 3, 4 andwhere an amplification and analysis of the circuitry therein willfacilitate the understanding of this method of remotely controlling andactivating a seismic vibrator.

ENCODER Looking now at the encoding section, as shown in FIG. 3, thefunction of the encoder is to generate a coded tone to start atprecisely the same time the remote generators or computers at thevibrators and at the recording-processing equipment. It is to beunderstood that while only one remote vibratory system is depicted inthe drawings, a multiplicity of such stations may be used and excited byencoder analog-digital control signal generator 42. The explanation ofonly one system is for the sake of clarity and simplicity and theremaining remote stations are identical in both makeup and operation.

Referring to FIG. 3, a stable oscillator 60 generates a tone, thefrequency of which is approximately equal to the center frequency of theband pass filters in the decoder section which will be subsequentlydiscussed.

The polarity of the tone is reversed by a unity gain inverting amplifier62, so that both polarities of the tone are presented to an electronicswitch 64. In the absence of a commence signal, electronic switch 64connects the non-inverted tone to the A contact of a relay 66. When acommence signal is present, the inverted tone is connected to the Acontact of relay 66.

The timing sequence starts when a commence pulse fromrecording-processing equipment 14 is presented to a flip-flop 68 throughlead 45. The output of flip-flop 68 energizes relay 66 connecting thetone to transmission line or wire 58. Contacts B of relay 66 may be usedto key a transmitter over line 50. Flip-flop 68 also enables an AND gate70, allowing pulses from a precision clock 72 to increment counter 74.

Substantially 750 milliseconds (ms) after the commence pulse, signalsfrom counter 74 enables a second AND gate 76, causing electronic switch64 to reverse the polarity of the tone. At 8 ms later, a third AND" gate78 is enabled, generating a pulse on lead 47 to a local control signalgenerating section, which starts to generate a unique signal.

One second after the commence pulse, a signal from counter 74 resetsboth counter 74 and flip-flop 68. This returns the encoder to itsoriginal state, preparing it for the next commence pulse.

DECODER Turning now to FIG. 4, there is shown the circuitry of a typicaldecoding section 54. The decoding section receives at the input thereofthe coded tone picked up by antenna 35, decodes such tone, and presentsa start pulse to the control signal generating section 48B.

The input signal, consisting of the tone code from encoding section 46and possibly unwanted noise and interference pickup during transmission,is applied to a sharp filter 80 and to a broad filter 82. Virtually allof the unwanted noise and interference is removed by sharp filter 80, sothat its output contains only the transmitted tone, shifted in phase bya constant but uncertain amount. This filtered tone is applied to theinput of phase shifter 84, a device which shifts the phase of the toneby any amount from 0 to 360 depending on the level of the controlvoltage. The output of phase shifter 84 is connected to squaringamplifier 86, a high gain amplifier which converts the sinusoidal toneto a fixed amplitude square wave having the same crossover times.

The broad filter 82 rejects much of the unwanted noise and interferencefrom the input signal and has a sinusoidal output which is converted bya second squaring amplifier 88 to a fixed amplitude square wave havingthe same crossover time.

The fixed amplitude square waves from squaring amplifiers 86 and 88 aremultiplied together by multiplier 90. If relay 92 is closed the outputof multiplier is filtered by a long time constant integrator in a resetamplifier 94. The resulting DC voltage is dependent on the phaserelationship between the inputs to multiplier 90. If this phaserelationship is 0 the voltage is maximum positive, decreasing through 0volts at 90 to maximum negative at The sum of this DC voltage and thevoltage from an initial reset level control 96 is connected to thecontrol voltage input of phase shifter 84. If the phase relationshipbetween the inputs to multiplier 90 is other than 90, the DC voltagefrom reset amplifier 94 will cause the phase introduced by phase shifter84 to change until the phase relationship is 90. At this time the DCoutput of multiplier 90 is zero, so the control voltage from resetamplifier 94 no longer changes and the phase between the outputs ofsquaring amplifiers 86 and 88 is locked in" at 90. Relay 92 may now beopened and the phase will remain 90 for a short period of time becausethe output of the long time constant integrator in reset amplifier 94cannot change rapidly.

The output of squaring amplifier 86 is integrated in 90 phase shifter 98to produce a triangular wave with crossovers delayed 90 from thecrossovers of the square wave input. This triangular wave is convertedto a fixed amplitude square wave by a third squaring amplifier 100, thecrossovers of which occur in phase with the crossovers of the trianglewave; therefore the output of squaring amplifier 100 is a square wave 90out of phase with the output of squaring amplifier 86.

Since the phase relationship between the outputs of squaring amplifiers86 and 88 has been corrected to 90, the phase relationship between theoutputs of squaring amplifiers 88 and 100 is 180. Thesetwo outputs aremultiplied together in a multiplier 102. A filter 104 eliminates all butthe DC component of this output. Since the inputs to multiplier 102 are180 out of phase, the DC component of the output is most negative. Withthe frequencies, bandwidths, and time constants used, about 200 ms fromthe start of the tone is required for the phase between the inputs tomultiplier 102 to lock in at 180.

An amplifier 106 increases the level of the sinusoidal tone from theoutput of sharp filter 80. A detector-filter 108 produces a DC voltageproportional to the magnitude of the tone and indicates the presence ofa tone. Since sharp filter 80 removes almost all noise and interferencefrom the input, only the desired tone can cause an output fromdetector-filter 108 of sufficient magnitude to be sensed by thresholdlevel sensors 110 and 112.

When the tone is first applied to the input the change in state ofthreshold level sensor 110 provides a pulse to trigger a monostablemultivibrator 114, which remains set for about 400 ms. During this 400ms relay 92 is closed, and the previously described phase lock loop hasadequate time to set the phase relationship between the inputs tomultiplier 102 to 180. When monostable multivibrator 114 times out after400 ms relay 92 opens and a second monostable multivibrator 116 istriggered. It remains set about 400 ms.

Exactly 750 ms after the tone starts, the phase thereof isinstantaneously changed 180. The input to multiplier 102 derived fromsharp filter 80 cannot change phase rapidly due to the extremely narrowbandwidth of this filter. The input to multiplier 102 derived from broadfilter'82 can change phase much more rapidly because of the widerbandwidth. As a result the phase relationship between the inputs tomultiplier 102 moves rapidly from 180 to almost after the input phasereversal, and then slowly moves back to 180. This causes the voltagefrom filter 104, which is most negative before the input phase reversal,to rapidly ramp to its most positive excursion and then slowly ramp backdown to its most negative value.

A comparator 118 senses the time at which this voltage crosses zero andprovides an output pulse at this time.

A reference level control 120 may be used to adjust the threshold levelof comparator 118 slightly about 0 volts, thus providing a slight amountof control over the time at which a pulse occurs on the output of thecomparator. With the frequencies, bandwidths, and time constants used,level control may be adjusted so that the pulse from comparator 118occurs 8 ms after the input time phase reversal, or 758 ms after thestart of the tone. At this time monostable multivibrator 116 is stillset so it is supplying an output.

The tone is still present at this time so threshold level sensor 112 issupplying an output. Thus 758 ms after the start of the tone fromencoding section 46, all of the inputs to an AND"gate 122 are qualifiedto generate a start pulse at the output thereof which is directly cou-CONTROL SIGNAL GENERATING SECTION CIRCUITRY Turning now to FIG. 5, thereis schematically shown the circuitry of a typical control signalgenerating section 48B. The circuitry of generating section 48A isidentical except the input thereof is coupled to encoding section 46rather than decoding section 54. The purpose of control signalgenerating section is to locally compute the unique signal to be sentinto the earth by vibrator 30. Mathematically this unique signal isrepresented by the sweep equation sine (K i K r).

The start pulse received over wire 124 from decoder section 54 sets aflip-flop 126. The output of flip-flop 126 enables an AND gate 128,allowing pulses from a precision clock 130 to increment two-stagecounter 132, there beginning the timing sequence. Two-stage counter 132provides pulses to an eight-stage counter 134 at a rate of 256,000pulses per second. Eight-stage counter 134 provides the timinginformation required to initiate the commands which must be executedevery l/l024 seconds. Eight-stage counter 134 increments a l4-stagecounter 136 at a rate of 1,024 pulses per second. l4-stage counter 136provides the 14 bit time word used to compute the value of the sweepequation and also provides timing information required to control theoperation of the sweep generator.

Fourteen-stage counter 136 provides signals to a delay switch 138indicating 0 second, 0.5 second, and 1 second after the start pulse. Thesignal from AND gate 140 through OR gate 141 resets all of the countersto zero and sets a taper flip-flop 142; fourteen stage counter 136 nowstarts generating digital time words to be used in computation of theunique signal starting with zero.

Taper flip-flop 142 has a step function output which is integrated by anintegrator 144 to produce a linear ramp function. The time constant ofintegrator 144 is adjusted so that the desired taper time is requiredfor the ramp to reach full voltage.

The linear ramp function is converted to a cosinusoidal function bycosine shaper 146. This rounding of the corners of the linear rampfunction by cosine shaper 146'has been found desirable as sucheliminates spurious frequencies. An amplified low source impedanceversion of the cosine ramp is obtained from an amplifier 148, and thisvoltage is used as the reference voltage to a D/A converter 150. The D/Aconverter 150 is of the multiplying type and the output thereof isproportional to the value of this voltage. As a result, the envelope ofthe sweep from the D/A converter increases cosinusoidally from 0 at thebeginning of the sweep to full value at the end of the taper time andremains at full value until taper flip-flop 142 resets.

A sweep length switch register 152 receives digital time informationfrom fourteen stage counter 136 and produces an output when an integernumber of seconds have elapsed. The number of seconds, from 1 to 15, isselected by the setting of the switches in the register thereof. A tapertime switch 154 receives signals from fourteen-stage counter 136indicating when 500 ms and 750 ms have elapsed since an integer numberof seconds have passed. An AND gate 158 requires inputs from both tapertime switch 154 and sweep length switch register 152. Thus AND" gate 158produces an output either 500 or 750 ms after the integral number ofseconds set in the switch register.

The output from AND" gate 158 resets taper flipflop 142 which causes theoutput of integrator 144 to ramp down linearly. The output of cosineshaper 146 and amplifier 148 follows by also ramping downcosinusoidally. The envelope of the output of the D/A converter 150 thendecreases cosinusoidally to zero during the taper down time, which isthe time required for the output of integrator 144 to ramp to zero; thetaper down time therefore being equal to the taper up time. The outputof AND gate 158 is delayed by a delay element 160, so that the output ofthe delay element is valid for about 1 ms after the next integral secondhas elapsed. This output is applied to an AND gate 162. A signal froml4-stage counter 136 indicating when the next integral second haselapsed is also applied to AND gate 162, to produce therefrom an outputone second after the integral second indicated by sweep length switchregister 152.

The output of AND gate 162, which occurs at the same time that theoutput of amplifier 148 has tapered the output of D/A converter 150 tozero, resets the start flip-flop 126 and sets all counters to zerothrough OR gate 141, thus preparing the device for another initializingstart pulse.

The previously described cycle occurs once every sweep. The cycle to bedescribed next occurs once every l/l024 seconds, or once every sampleinterval, and is the sequence of operations required to compute thevalue of the sweep equation for each sample interval.

Eight-stage counter 134 provides an 8 bit digital time word whichindicates the progression of time within the l/l024 second sampleinterval to a resolution of 256 parts.

An instruction sequence decoder 164 uses this time word to generateseveral outputs, each providing a pulse at a sequential point in time.These outputs are combined in an instruction generator 166 to supplycontrol signals to operate certain arithmetical units. Pulses frominstruction sequence decoder 164 will be numbered in chronologicalorder. For example, the first pulse will be called time 1.

Time 1 generates a Kl enable pulse. Depending on which switches of a KIswitch register 168 are on, data will be presented to certain bits ofthe ACC input of a multiplier 170. Time 1 also generates an ACC loadpulse, which enters the data into the ACC input of multiplier 170 andstores it there. Time 1 also generates a pulse to clear the outputregister of the multiplier.

Time 2 generates a T-enable pulse which enables a gate 172 which in turnallows the 14 bit time word from l4-stage counter 136 to be presented tothe MO inputs of multiplier 170. Time 2 also generates an M load pulsewhich enters the time word into the MO input of the multiplier 170 andstores it there.

Time 3 l8 generate 16 pulses which cause multiplier 170 to sample eachsuccessively more significant bit of the data in the MO input and, if itis a one, to add the data in the ACC input to the contents of themultipliers output register before shifting the data in the outputregister 1 bit to the right. This causes multiplier 170 to multiply thecontents of the MO input by the contents of the ACC input and leaves theproduct in the multiplier output register.

Time 19 generates a pulse which makes the output of multiplier availableto the input of an adder/subtractor 174 and another pulse which entersthis data into the input of the adder/subtractor.

Time 20 generates a pulse which clears the multipliers output.

Time 21 generates a T-enable pulse, enabling gate 172 and a gate 176 toallow all the 14 bit time word from l4-stage counter 136 to be presentedto the MO and ACC inputs of multiplier 170. Time 21 also generates MQload and ACC load pulses which enter the time word into the MQ and ACCinputs of multiplier 170 and stores it there.

Time 22-37 cause multiplication to occur as before. The product I isleft in output of multiplier 170.

Time 38 generates a pulse which makes the output of multiplier 170available to the ACC input thereof. Time 38 also generates a pulse whichenters this data into the ACC input of multiplier 170 and stores itthere. Time 39 clears the output of the multiplier.

Time 40 generates a K2 enable pulse. Depending on which switches in a K2switch register 178 are on, voltage will be presented to certain bits ofthe MO input of multiplier 170. Time 40 also generates an MQ load pulse,which enters this data into the MO input. Time 40 also generates a pulsewhich clears the output of adder/subtractor 174.

Times 41-56 cause multiplication to occur as before. The product K21 isleft in the output of multiplier 170.

Time 57 generates an OUT-AC pulse, which causes the contents of theinput of adder/subtractor 174 to be transferred to the output thereof.

Time 58 generates a MUL STR pulse, which makes the output of multiplier170 available to the input of adder/subtractor 174. Time 58 alsogenerates an OUT- load pulse which enters this data into the input ofadder/subtractor 174. Time 58 also generates an OUT- AC pulse, whichcauses the input of an adder/subtractor 174 (K21 to be added to theoutput thereof (Klt), with the result left in the output. However, ifUP- DOWN switch is on, time 58 also generates an OUT-AS pulse, whichcauses K2t to be subtracted from Klt. So depending on the position ofup-down switch 170, the output of now adder/subtractor 174 contains a 9bit word representing the angle (Klt K22 The MSB of this 9 bit wordrepresents the sign of the trigonometric sine of this angle. At time 59the MSB is stored in sign selector 182, a one bit storage register. Thesecond MSB indicates whether the angle is 0, 1r/2 or 1r 31r/2, or 17/21r or 317/2 211-. If the second MSB is 0, the 7 LSBs represent an anglebetween 0 17/2 of which the trigonometric sine may be taken directly. Ifthe second MSB is l, the 7 LSBs must be complimented by complimentenable 177 before the trigonometric sine may be taken. So at time 59 thesecond MSB of the output of adder/subtractor 20 is sampled. If it is 1,three additional instruction times, 60-62, are generated. I

Time 60 generates an OUT-STR pulse which presents the 7 LSBs of theoutput of adder/subtractor 174 (containing Klt i K2t to the input ofadder/subtractor 174. Time 60 also generates an OUT load pulse whichenters this data into the input of adder/subtractor 174.

Time 61 generates an OUT-RA pulse which clears the output ofadder/subtractor 174.

Time 62 generates a pulse which borrows 1 from the LS8 of the data inadder/subtractor 174.

Time 62 also generates a pulse which causes the input adder/subtractor174 to be subtracted from zero. These operations change all zeros toones, and vice versa, which in effect, compliments the 7 bit number inadder subtractor 20s output register.

The seven least significant bits of this 8 bit number represent an anglebetween and 90 whose trigonometric sine is equal to the absolute valueof the desired trigonometric sine, that is, of the angle Klt i K2t Time63 makes the output of adder/subtractor 174 available to the input ofread-only-mernory 184. Readonly-memory 184 is a 1,024 bit memorypreprogrammed as a sine look-up table. That is, when addressed by adigital word representing an angle, its output supplies a digital wordrepresenting the trigonometric sine of that angle. So the output ofread-onlymemory 184 is now a digital word representing the absolutevalue of sin [Klt K21 Time 63 also generates a pulse which enters datafrom read-only-memory 184 and sign selector 182 into the input of D/Aconverter 150 and stores it there. D/A converter 150 generates an analogvoltage proportional to the product of this digital information and thereference voltage from amplifier 148. This analog voltage remainsconstant for 1/1024 seconds, until time 63 of the next sample interval.So that output of D/A converter 150 is a stepwise approximation of theswept frequency sinusoidal wave, sine [Klt i K2t Linear phase filter 186removes the steps from this wave by fil-. tering out high frequencies.The output of linear phase filter 186, though slightly delayed in time,is an accurate representation of sine [K t i K 2 Sine [K,! i K 1 asbefore mentioned is the desired sweep equation for the unique signal.The output of linear phase filter 186 is coupled to control electronicsvia lead 56, which in turn passes the signal to vibrator 30 forintroduction into the earth.

Further understanding of the terminology and circuitry of this inventionis facilitated by reference to the diagrams and internal couplings ofthe numerous electronic components as set forth in the literaturedistributed by the manufacturers thereof. The following is a list oftypical manufacturers as well as the respective catalog number of eachcomponent.

LEGEND REFERENCE NUMBER 66 Hart Advance DPDT relay. 68 Flip-flop TexasInstrument No.

70 "AND" gate-V4 of Texas Instrument No. 7400.

72 l.048576 MHZ crystal oscillator-International Crystal Corp.

74 22 stage counter consisting of A of Texas Instrument No. 7493 inseries.

AND gate-V of Texas Instrument No. 7400.

AND gate-V4 of Texas Instrument No. 7400.

DECQDING CIRCUIT Electro-magnetic resonator-Motorola No. KIOOOA.

Parallel inductor-capacitor tuned circuit, Q Figure of merit ofapproximately 70.

2 stages of variable phase shift,

each composed of transistor (2N3900A) 4-phase splitter driving fixedcapacitor and voltage variable resistor (Siliconix No. VCR3P FET).

National Semiconductor LM3I 1 comparator.

National Semiconductor LM3I I comparator.

Exclusive OR gate-V4 of Texas Instrument No. 7486.

SPDT relay-Magnecraft.

Integrating amplifier National Semi No. LM307 withresistance-capacitance feedback network.

Potentiometer-Clarostat No.

Resistance-capacitance integrator.

National Semi No. LM3II comparator.

Exclusive OR" gate-Va of Texas Instrument No. 7486.

Resistance capacitance low pass filter.

GE 2N3900A transistor in conventional class A amplifier circuit.

Texas Instrument No. IN270 diode rectifier and resistance capacitancelow pass filter.

Conventional Schmitt trigger circuit, using Motorola MP5 6518 andMPS3704 transistors.

Conventional Schmitt trigger circuit, using Motorola MP8 l 8 and MPS3704transistors.

Fairchild No. 960I.

Fairchild No. 9601.

National Semi No. LM3ll comparator.

Potentiometer-Clarostat No.

3 input AND gate-96 of Texas Instrument No. 7410.

CONTROL SIGNAL GENERATING SECTION Flip-flop% of Texas Instrument ANDgate-Va of Texas Instrument No. 7400.

1.048576 MHZ Crystal oscillator-International Crystal Corp.

2 stage binary counter- A of Texas Instrument 7493.

8 stage binary counter-2 Texas Instrument No. 7493.

I4 stage binary counter-2 Texas Instrument No. 7493 in series.

Switch SP'IT-ALCO.

AND Gate-/4 of Texas Instrument No. 7400.

FIip-fiop% of Texas Instrument Active integrator, composed of Nationalsemi-conductor LM307 adjustable input resistor, and feedback capacitor.

Cosine shaper network composed of diodes, such as Texas Instrument No.IN457A and selected resistors.

Amplifier National Semiconductor No. LM307.

D/A converter composed of Fairchild 3750, Beckman 8l2 ladder network,Nat semi-conductor LM307 offset correction stage, LM307 sign selectioninverter stage, drive by VCRBP (Siliconix) FCT switch.

Group of 4 SPDT switches ALCO.

Switch SPDT-ALCO AND Gate-V4 of Texas Instrument No. 7400.

Delay element (resistance-capacitance network),

AND" Gate-V4 of Texas Instrument No. 7400.

l of 24 decoder-Motorola 3 MC4038's.

I multiple input OR gate for each command Texas Instrument 7410.

Group of 7 SPST switches ALCO.

Composed of 4 8 bit parallel accumulators-Fairchild No. 3800.

AND" Gate-V4 of Texas Instrument No. 7474.

8 bit parallel accumulator-Fairchild 3800.

AND Gate Tl, V4 of Texas Instrument No. 7474.

One bit storage element-J5 of Motorola No. 7479 and, AND" Gate-TexasInstrument No. 7400.

Group of 7 SPST switches ALCO.

Switch SPST-ALCO.

One bit full adder-Texas Instrument No. 7480 and one bit storageelementof Motorola No. 7479.

I024 bit read only memory programmed as sine look-up table-PHILCO No.PMSI024C.

Active 3 pole Paynter linear phase filter composed of Nat. semi LM307and resistance-capacitance network.

During the detailed description of the preferred embodiments specificlanguage has been used for the sake of clarity, however, it is to beunderstood that the words used are not words of limitation and includeall equivalents which operate in a similar manner to accomplish asimilar purpose.

It will be clear that the control signal generating sections 48A and 48Bare identical. Generating section 48B gets its input from decodingsection 54 since it is at a remote point and must receive its startingsignal via radio in the form of a coded signal. On the other hand thegenerating section 48A is adjacent to both the recording-processingequipment 14 and the encoding section, and can receive its startingsignal as a facsimile pulse via line 47 directly from gate 78, which istimed to occur at the same time as the pulse generated in the decodingsection. This facsimile pulse is timed to be simultaneous with the pulsegenerated in the decoding section, and may be called a facsimile of thedecoded, coded signal. More generally, the equipment at the encodinganalog-digital control signal generator can comprise an encoding section46, and a decoding section 54 and a control signal generating section48A.

Also, while we have shown a purely digital means for directly generatingthe sweep signal to be supplied to the vibrator, it will be clear that,as shown in FIG. 1 of U.S. Pat. No. 3,460,648, the sweep signal can belocally generated digitally, or locally stored as a digital or analogrecord, as is well known in the art.

Also, while we speak of phonographically recording the received uniquesignals, we include any means of recording such signals in such formthat the recorded signals can be played back to regenerate the receivedunique signals. The preferred method of phonographically recording is bymeans of a magnetic recorder.

What is claimed:

1. In a seismic system having at a first location at least one vibratorcapable of imparting a unique signal into the earth, having at a secondlocation a recording means and a detector array electrically coupled tosaid recording means, said detector array adapted to receive said uniquesignal after transmission through the earth from said first location, amethod of providing a selected unique signal to each of said first andsecond locations, comprising the steps of:

a. generating a selected short-duration coded signal,

b. simultaneously transmitting said coded signal to said first andsecond locations,

0. generating a selected unique signal responsive to the arrival at saidfirst location of said coded signal, and

d. generating an identical selected unique signal responsive to thearrival at said second location of said coded signal.

2. A method as in claim 1 in which said step of generating said selectedunique signal includes the step of decoding said coded signal.

3. A method as in claim 1 in which said coded signal transmitted to saidsecond location comprises a facsimile of the decoded, coded signal.

4. A method as in claim 1 in which the step of generating a selectedshort-duration coded signal is responsive to the step of transmitting acommence pulse from said recording means.

5. A method as in claim 1 including the additional steps at said firstlocation of transmitting said unique signal to said vibrator andimparting said unique signal into the earth.

6. A method as in claim 1 including the additional step at said secondlocation of phonographically recording said unique signal.

7. A method as in claim 6 in which said step of phonographicallyrecording said unique signal comprises magnetically recording saidsignal.

8. In a seismic system having at a first location at least one vibratorcapable of imparting a unique signal into the earth, having at a secondlocation a recording means, and a detector array electrically coupled tosaid recording means, said detector array adapted to receive said uniquesignal after transmission through the earth from said first location,the improvement comprising:

a. means for generating a selected short-duration coded signal, and afacsimile signal,

b. means for simultaneously transmitting said coded signal to said firstlocation, and said facsimile signal to said second location,

c. means at said first location for receiving and decoding said selectedcoded signal,

d. means at said first location responsive to said decoded, selectedcoded signal for generating a selected unique signal, and

e. means at said second location responsive to said facsimile signal forgenerating an identical unique signal, said facsimile signal occurringsimultaneously in time with said decoded, selected coded signal.

9. A seismic system as in claim 8 including means at said first locationfor transmitting said selected unique signal to said at least onevibrator, whereby said unique signal is imparted into the earth.

10. A seismic system as in claim 8 including means at said secondlocation to phonographically record said unique signal.

11. A seismic system as in claim 8 including means at said secondlocation for generating a commence pulse and means for transmitting saidcommence pulse to said coded signal generating means.

12. A seismic system as in claim 8 in which said means for generatingsaid unique signal comprises:

' a start flip-flop having a set input, a reset input and an output;said set input receiving said signal from an encoder;

a two-input AND gate, one of said inputs being electrically coupled tosaid output of said start flipp;

a precision clock electrically coupled to the second input of said ANDgate;

a two-stage counter having an enable input; reset input; and output;said set input being electrically coupled to said output of said ANDgate;

an eight-stage counter having an enable input, reset input and output,said enable input being electrically coupled to the output of saidsecond stage counter;

a l4-stage counter having an enable input, reset input and output; saidenable input being electrically coupled to the output of saideight-stage counter;

a sweep length switch register having an output;

a manual switch having an output;

a second two-input AND gate; one of said inputs being electricallycoupled to said sweep length switch register and the other of said inputsignals being connected to said manual switch;

a first relay having an input connected to the output of said second ANDgate, said relay having an output;

a third two-input AND gate having one input thereof electrically coupledto the output of said relay and a second input being electricallycoupled to the output signal of said l4-stage counter, the output ofsaid third AND gate being electrically coupled to the reset input ofsaid start flip-flop;

a taper flip-flop having an input electrically coupled to said secondtwo-input AND gate and further having dual outputs;

an integrator having an input and output, said input being electricallycoupled to the output of said taper flip-flop;

a cosine shaper having an input electrically coupled to the output ofsaid integrator and further having an output;

a digital-to-analog converter having an input electrically coupled tothe output of said cosine shaper;

a manual switch;

a fourth two-input AND gate, one of the inputs being electricallycoupled to said manual switch, and the other being electrically coupledto the output of said start flip-flop, the output of said fourth ANDgate being connected to said taper flipp;

a fifth dual input AND gate, one input being electrically coupled to theoutput of said W second AND gate and the other being electricallycoupled to the output of said fourth AND gate; the output of said ANDgate being connected in parallel to said second-stage, eight-stage, and14-stage counters;

an instruction sequence decoder having a plurality of inputselectrically coupled to the outputs of said eight-stage counter andhaving a plurality of outputs; an instruction-in generator having aplurality of inputs electrically coupled to said plurality outputs ofsaid instruction sequence decoder.

1. In a seismic system having at a first location at least one vibratorcapable of imparting a unique signal into the earth, having at a secondlocation a recording means and a detector array electrically coupled tosaid recording means, said detector array adapted to receive said uniquesignal after transmission through the earth from said first location, amethod of providing a selected unique signal to each of said first andsecond locations, comprising the steps of: a. generating a selectedshort-duration coded signal, b. simultaneously transmitting said codedsignal to said first and second locations, c. generating a selectedunique signal responsive to the arrival at said first location of saidcoded signal, and d. generating an identical selected unique signalresponsive to the arrival at said second location of said coded signal.2. A method as in claim 1 in which saiD step of generating said selectedunique signal includes the step of decoding said coded signal.
 3. Amethod as in claim 1 in which said coded signal transmitted to saidsecond location comprises a facsimile of the decoded, coded signal.
 4. Amethod as in claim 1 in which the step of generating a selectedshort-duration coded signal is responsive to the step of transmitting acommence pulse from said recording means.
 5. A method as in claim 1including the additional steps at said first location of transmittingsaid unique signal to said vibrator and imparting said unique signalinto the earth.
 6. A method as in claim 1 including the additional stepat said second location of phonographically recording said uniquesignal.
 7. A method as in claim 6 in which said step of phonographicallyrecording said unique signal comprises magnetically recording saidsignal.
 8. In a seismic system having at a first location at least onevibrator capable of imparting a unique signal into the earth, having ata second location a recording means, and a detector array electricallycoupled to said recording means, said detector array adapted to receivesaid unique signal after transmission through the earth from said firstlocation, the improvement comprising: a. means for generating a selectedshort-duration coded signal, and a facsimile signal, b. means forsimultaneously transmitting said coded signal to said first location,and said facsimile signal to said second location, c. means at saidfirst location for receiving and decoding said selected coded signal, d.means at said first location responsive to said decoded, selected codedsignal for generating a selected unique signal, and e. means at saidsecond location responsive to said facsimile signal for generating anidentical unique signal, said facsimile signal occurring simultaneouslyin time with said decoded, selected coded signal.
 9. A seismic system asin claim 8 including means at said first location for transmitting saidselected unique signal to said at least one vibrator, whereby saidunique signal is imparted into the earth.
 10. A seismic system as inclaim 8 including means at said second location to phonographicallyrecord said unique signal.
 11. A seismic system as in claim 8 includingmeans at said second location for generating a commence pulse and meansfor transmitting said commence pulse to said coded signal generatingmeans.
 12. A seismic system as in claim 8 in which said means forgenerating said unique signal comprises: a start flip-flop having a setinput, a reset input and an output; said set input receiving said signalfrom an encoder; a two-input ''''AND'''' gate, one of said inputs beingelectrically coupled to said output of said start flip-flop; a precisionclock electrically coupled to the second input of said ''''AND'''' gate;a two-stage counter having an enable input; reset input; and output;said set input being electrically coupled to said output of said''''AND'''' gate; an eight-stage counter having an enable input, resetinput and output, said enable input being electrically coupled to theoutput of said second stage counter; a 14-stage counter having an enableinput, reset input and output; said enable input being electricallycoupled to the output of said eight-stage counter; a sweep length switchregister having an output; a manual switch having an output; a secondtwo-input ''''AND'''' gate; one of said inputs being electricallycoupled to said sweep length switch register and the other of said inputsignals being connected to said manual switch; a first relay having aninput connected to the output of said second ''''AND'''' gate, saidrelay having an output; a third two-input ''''AND'''' gate having oneinput thereof electrically coupled to the output of said relay and asecond input being electrically coupled to the output signal of said14-sTage counter, the output of said third ''''AND'''' gate beingelectrically coupled to the reset input of said start flip-flop; a taperflip-flop having an input electrically coupled to said second two-input''''AND'''' gate and further having dual outputs; an integrator havingan input and output, said input being electrically coupled to the outputof said taper flip-flop; a cosine shaper having an input electricallycoupled to the output of said integrator and further having an output; adigital-to-analog converter having an input electrically coupled to theoutput of said cosine shaper; a manual switch; a fourth two-input''''AND'''' gate, one of the inputs being electrically coupled to saidmanual switch, and the other being electrically coupled to the output ofsaid start flip-flop, the output of said fourth ''''AND'''' gate beingconnected to said taper flip-flop; a fifth dual input ''''AND'''' gate,one input being electrically coupled to the output of said second''''AND'''' gate and the other being electrically coupled to the outputof said fourth ''''AND'''' gate; the output of said ''''AND'''' gatebeing connected in parallel to said second-stage, eight-stage, and14-stage counters; an instruction sequence decoder having a plurality ofinputs electrically coupled to the outputs of said eight-stage counterand having a plurality of outputs; an instruction-in generator having aplurality of inputs electrically coupled to said plurality outputs ofsaid instruction sequence decoder.